The present invention relates to magnetic storage systems. In particular, the present invention relates an addressing format for a magnetic disc storage system.
Magnetic disc drive systems have become widely accepted in the computer industry as a cost effective and reliable form of data storage. The advantages of disc drive technology over other means of data storage include increased data transfer rate and storage capacity. These two goals are motivating forces of disc drive technology.
In a magnetic disc drive storage system, a magnetic disc rotates at high speed while a magnetic read/write head "flies" over the surface of the rotating disc. As the disc rotates, aerodynamic properties cause the magnetic head to glide on a cushion of air suspended over a surface of the disc. The storage disc carries information on concentric data tracks. Information can be retrieved from the disc surface by moving the magnetic head between data tracks.
In general, there are two types of actuators which are used to position the magnetic head over the disc surface. These two types are linear and rotary actuators. A linear actuator moves back and forth linearly, in a radial direction from the center of the rotating disc. Rotary actuators, however, require less space than linear actuators. A rotary actuator works much like a tone arm on a record player. A rotary actuator positions the magnetic head along an arc over the surface of the magnetic disc. This can introduce a skew angle between the magnetic head and the data tracks.
A dual gap magnetic head uses a read gap for reading information and a write gap for writing information. Dual gap magnetic heads allow magnetic head designers to optimize the performance of the read gap for reading information and the write gap for writing information. For example, a magnetoresistive head uses a read gap for a magnetoresistive readback element and a write gap for inductively writing magnetically encoded information. Design constraints require that the two gaps be physically separated from each other.
Performance of a dual gap magnetic head can be optimized for a particular data track on a magnetic disc by aligning the write gap with the data track when writing information and aligning the read gap with the data track when reading information. Using a linear actuator, this is not a problem. However, a rotary actuator introduces a skew angle between data tracks and that axis of a dual gap head which provides optimum performance. For example, a dual gap head having 320 micro inches of separation between the two gaps and a 9.degree. skew angle with a data track will have an offset of 50 micro inches.
An actuator controller which controls positioning of the actuator can compensate for the skew angle introduced by a rotary actuator. Still, the skew angle significantly limits the time required to write information. This limitation is due to the format used in storing data on data tracks of the magnetic disc. Typically, a data track is formatted with an address field followed by a data field. The address field contains information which relates to head position and data field integrity, and therefore the address field must be read before the data field is written. Prior to writing information in a data field, the magnetic head must be repositioned because of the offset introduced by the skew angle. This can require from 0.5 micro seconds to 1.5 micro seconds. In fact, in a high speed magnetic disc storage system the head may be required to wait an entire revolution of the disc before writing information.